For individuals that are sensitive to airborne allergens, the severity of an allergic response is often not linearly proportional to the concentration of allergens in the air they breathe. Often there is little or no response unless exposure exceeds the particular threshold concentration that triggers a cascading symptomatic reaction. To prevent onset of such symptoms, it is therefore only necessary to limit exposure to some concentration below that threshold. Presently available personal respirators vary in both filtration efficiency and degree of user comfort, and these qualities are generally inversely proportional to each other.
Presently available personal respirators also commonly incorporate a one-way valve feature that allows free expiration, to reduce required breathing effort and to minimize inspiration of previously expired carbon dioxide. Such valve features add weight, bulk, and cost.